Heavy slabs and sheets of material are usually stored and stacked upright-standing. Handling of this type of materials often entails use of lifting clamps that grip the sheet (hereinafter used generically to also encompass slabs and other planar objects) at its upper edge for hoisting. Consequently, it is convenient in the following description to use reference terminology such as ‘vertical’, ‘horizontal’, ‘upper’, ‘lower’, ‘inclined’ and similar when describing operation, components and relative location of components or parts of such clamping devices. Bearing in mind that these devices may also be used in a ‘horizontal’ or other orientation, eg as a simple clamp or a haulage attachment, it will immediately be appreciated that unless otherwise clear in the context, such reference terms, also when these appear in the appended claims, are not to be regarded as limiting of the scope of the invention.
A lifting clamp of the type with which the present invention is concerned is known from U.S. Pat. No. 5,893,595 (Corbett). The Corbett lifting clamp includes a rigid frame comprising vertical, parallel spaced-apart side plates, the upper ends of which are rigidly secured together by four tubular cross-members. The lower portion of one of the side plates is angled away in downward orientation from the other side plate, the latter providing a fixed clamping jaw of the device. A vertically extending plate is mounted for horizontal sliding movement on the cross-members between the side plates and provides a movable jaw of the device. An actuator carriage which is disposed for guided vertical up and down movement, is located between the movable jaw and the lower, angled portion of the frame side plate, whereby different sets of rollers of the carriage respectively engage the facing surfaces of the movable jaw plate and the angled portion of the frame side plate. A lifting cable or chain is attachable to an extension lug at the carriage. In order to lift (or otherwise handle and clamp) sheet material, the device is placed over the upper edge of the sheet so that it is received between the fixed and movable jaw plates, the carriage is raised by lifting the strip member through pulling the lifting cable upwards, whereby the carriage travels on the angled frame side plate portion and displaces the movable jaw horizontally until it abuts on the facing surface of the sheet material. Upon increasing the upward pulling force, the sheet material is frictionally clamped for it to be lifted with the device.
In essence, clamping of the sheet material between the plate jaws is achieved by wedging the carriage between the frame side plate and the movable jaw, and the clamping force is maintained for as long as there is upward force being on the lifting cable.
Other types of clamps or grapples that employ the principle of a wedge/clamping element being used to impart movement to a clamping member are known for example from U.S. Pat. No. 2,387,408, U.S. Pat. No. 3,197,250 and U.S. Pat. No. 3,524,670.
One disadvantage of the Corbett clamp is that gripping force application unto an uneven sheet, or sheets which do not have properly parallel faces, is compromised. This stems from the fact that the movable clamp jaw is restricted in its degrees of freedom of movement to horizontal sliding movement only along the lower guide rails that rigidly join the frame sides together. The vertically depending jaw is unable to adjust itself into full plane-parallel contact with divergent sheet faces. This results in a reduced clamp engagement surface with the sheet and potential slippage problems on lifting. Equally, the reduced engagement surface area means higher and uneven pressure distribution, with resultant load concentration that can lead to sheet edge breakage or shattering of the sheets upon lifting.
One object of the present invention is to provide an improved lifting device of the aforementioned type which provides for an improved engagement of the clamping jaw surfaces with uneven-faced sheet material or other objects to be lifted.